scheldorf



March 17, 1964 M. w. scHELDoRF 3,125,757

CORONA ROO FOR ANTENNAS M. w. SCHELDQRF 3,125,757 CORONA ROO FOR ANTENNAS 2 Sheets-Sheet 2 m mam llllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllll d.

March 17, 1964 Filed Aug. 25, 1960 United States Patent C 3,125,757 CORONA RGD FR ANTENNAS Marvel W. Scheldorf, Western Springs, Ill., assigner to Andrew Corporation, Orland Park, Ill., a corporation of Illinois Filed Aug. 25, 196i), Ser. No. 51,887 14 Claims. (Cl. 343-726) This invention relates to a corona rod or lightning rod for use with antennas, and more particularly with antennas having vertically oriented conductors.

lIt is frequently important to prevent the appearance of corona currents in antennas, since corona discharge currents constitute broad band noise cre-ating serious problerns at all frequencies. Noise currents owing in an antenna due to corona (or lightning) excite the antenna at its resonance frequency, thus creating interference with transmitted or received signals. It has been found that the problem of corona noise becomes particularly substantial in the case of vertical antennas having, in addition to the fed element, a vertical element grounded at the lower end, since heavy corona currents may be caused to flow in the grou-nded conductor with this arrangement when the antenna is greatly elevated.

The use of a conventional lightning rod (a simple vertical conductor) adjacent to a Vertical antenna is destructive of the intended radiation pattern and further introduces great variations in the antenna input impedance with frequency, since the lightning rod and its associated ground lead will not only reradiate the signal, but also will have different electrical lengths for various frequencies, even ,over a narrow band, thus destroying the impedance bandwidth of the antenna.

The present invention provides a construction for diverting corona and lightning currents from the antenna without interfering with its radiation pattern or its impedance to `an appreciable degree. The corona or lightning rod of the present invention may be added as an auxiliary to existing antennas without substantial detuning. -In the present construction, there is provided a conductor which extends vertically to a height above the end of the vertical antenna, the lower end of the rod being grounded in a manner similar to a conventional lightning rod. However, in the present construction, although the lightning or corona rod acts in conventional fashion for direct voltages and currents and for those of most frequencies, it is so constructed that at the frequencies of interest, i.e., at and immediately adjacent to the resonant frequency of the antenna, it acts as if it were a series of mutually insulated end-to-end conductors, each of very short length compared to the wavelength of interest, and thus incapable of substantial energy absorption or reradiation and accordingly more or less inert as regards alteration of the radiation pattern or impedance of the antenna.

This is achieved by surrounding the rod with a series of cup-shaped conductor-s to form a series of coaxial quarter-wave chokes. By providing in each choke a fur- .ther cup-shaped conductor of a different diameter and inverted with respect to the dirst, the chokes are folded so that their external radiating surface is reduced in length. In addition, the annuli thus formed, between the sidewalls of the inner and cuter cup-shaped conductors forming the choke, and between the inner sidewall and the central rods, are packed with an insulator of dielectric constant substantially greater than unity to further reduce Ythe length `of each choke. Further reduction of length for the .given tuned frequency is obtained by proper selection of cup diameters. By making the ratio of the spacing of the inner cup wall from the center rod to the center rod diameter greater than the corresponding ratio in the coaxial space between the outer cup and ice the inner cup, each choke assembly appears effectively from its input end to have an input portion of relatively low impedance coaxial line followed by a shorted portion of substantially higher impedance coaxial line, thus producing quarter wave resonance at substantially smaller overall length than is required where the inner sleeve is placed at the geometric mean between the rod diameter and the outer sleeve or cup diameter to produce a quarter wave line of uniform impedance characteristics along its entire (folded) length.

For more complete understanding of the invention, reference is made to the single embodiment thereof shown in the annexed drawing, in which:

FIGURE l is a schematic line drawing of a corona or lightning rod made in accordance with the invention;

FIGURE 2 is a perspective View of an assembly of a particular vertical antenna and a corona rod, constructed in accordance with the invention.;

FIGURE 3 is a fragmentary sectional View showing the detailed inner construction of the corona rod shown in FIGURE 2; and

FiGUR-E 4 is an enlarged perspective View showing the manner of attachment and grounding of the lower end of the corona rod of FIGURE 2.

The invention is shown in its broader aspects in highly schematic form in FIGURE 1. The corona rod assembly is generally designated by the numeral 10. A central conductor rod 12 is surrounded by a series of end-to-end assemblies 14. Each assembly 14 has an inner cupshaped conductor l116 having the rod 12 extending in tightitting relation through the closed end thereof and soldered thereto and an outer cup-shaped conductor 18 of larger diameter, inver-ted with respect to the inner cup 16 and again having the rod 12 extending in tight-fitting relation through the closed end and soldered thereto.

The lip of the inner cup lr6 is spaced from the closed end of the outer cup `18 to produce a gap or passage at 20. The respective assemblies 14 are closely adjacent but slightly spaced by gaps Z2. The upper end Z4 of the rod l2 extends slightly beyond the uppermost choke assembly 14. The lower end of the rod 12 is grounded at 26.

It will be seen that each of the choke assemblies -14 constitutes a folded coaxial structure having a `shorfted termination portion constituted by the central rod 12 and the side wall of the cup i1'6, which are separated by the inner annulus 27, and an input portion constituted by the side walls of the inner cup 16 and the outer cup 18, which are separated by the outer annulus 28.

The structure as so far described may be employed to form a series of quarter wave chokes, each with a physical external length of approximately 1A; of a wavelength. It is found experimentally that the subdivision of the conducting rod 12 into pieces of this length (for the desired frequency), although substantially better than a simple rod, nevertheless produces serious effects upon pattern and impedance when placed in the vicinity of a vertical antenna. The results are generally similar when there are employed simple 1/8 wavelength (physical length) cups loaded with a dielectric having a constant of 4:0. However, when the physical length of the chokes is further substantially reduced to 1/16 wavelength or less, it is found that the rod can be placed in the immediate neighborhood of the vertical antenna with rapidly diminishing effect on its performance. Such fur-ther shortening may in theory be accomplished by loading a simple cup with certain high dielectric constant ceramics, etc., but from the economic standpoint is best achieved by combining the shortening achieved by the folded construction illustrated with that achieved by dielectric loading with such commonly available insulators as fiberglass. Where the latter, which has a dielectric constant of 4.0,

is employed, and the inner cup 16 is of a diameter which is the geometric mean between the diameters of the center rod 12 and the outer cup l18 (neglecting the thickness'of the cups), the two series sections of the coaxial structure haveV the same characteristic impedance, and Vthe overall length of earch choke is approximately lA; Wavelength. In accordance with the present invention, further shortening of the physical structure is obtained by properly mismatching the characteristic impedances of the two series sections.` This is done by so locating the inner cup that the characteristic impedance of the input (open-ended) section is lower than that of the shorted, or remote, section of the choke. As may be shown by theory, and verified experimentally, such mismatching produces quarter-wave tuning, vw'th any given dielectric, ata physical length substantially shorter than the physical length required for quarterdwave tuning where the segments of the line (folded or otherwise) are matched in characteristic impedance.

Such a mismatch is readily achieved by making the annulus 27 equal in thickness to the annulus 29. This structure, in addition to shortening the physical length of the choke segments, has the further great advantage that it eliminates the waste which is inherent in having annuli of different sizes, since in the latter case only one of the annuli can be designed for maximum economy of size and materials.

It will of course be observed that the folded choke assemblies may be designed in a manner dilferent from the mutually inverted cup arrangements shown in FIG- URE 1. For example, if the closed end of the cups 16 is replaced by an annular conductor connecting the ends of the sidewalls of the inner and outer cups, there results a structure which is generally similar, but in which the input annulus is the inner annulus, and the shorted or termination portion is the outer annulus. Such a construction, however, cannot be employed advantageously with annuli of equal thickness, since in this case the input segment of the line is of the higher impedance, thus requiring greater physical length for tuning to the same frequency than is required with equal size annuli in the device illustrated. Other variants, such as doubly-folded, rather than singly-folded, choke assemblies, may be devised, but are in general inferior to the structure illustrated because of the substantial additional cost and complexity introduced.

FIGURES 2 through 4 show in some detail a practical commercial embodiment of a device incorporating the teachings of the invention. As shown in FiGURE 2, the corona rod assembly is employed in conjunction with a folded quarter wave antenna generally designated by the numeral 29, consisting of a fed conductor 30 terminating at its top in a U-section 32, and at its bottom in a coaxial connector 34, which is mounted in and supported by a tubular mast 36. A reversely folded conductor 38 terminates at the top in the other arm of the U-section 32 and is grounded at its lower end 4@ to a plate 4l conductively mounted on the mast 36. Horizontal ground rods 42, mounted on the plate 41, form the ground plane or counterpoise of the antenna. It will be understood that the construction of the antenna itself, except in combination with the corona rod of the present invention, is not the invention herein disclosed, the particular antenna with which the corona rod is employed in the illustrated embodiment being selected for purposes of showing one commercial embodiment, and the manner of design of the radiating portion of the antenna being already known.

As will further be seen in FIGURE 2, the corona rod 10 has its upper end extending substantially above the upper end of the antenna proper and consists of the five choke assemblies 14 (indicated by dotting), terminating at the discharge point formed by the extending upper end 24 of the central rod. The central portion of the corona rod 10 is supported by a suitable insulating clamp 44.

The details of mounting and connection of the lower 4 end of the rod 10 are shown in FIGURE 4, in which it may be seen that mounting and grounding are done by means of a generally rectangular bar 46 horizontally apertured at 48 to fit over one of the ground rods 42, a slot 5t) extending from the end of the bar to the aperture 48 permitting clamping of the aperture 48 around the ground rod 42 by tightening of a bolt 52 and nut 54.

A similar but vertical slot 56 permits clamping of the lower end of the rod 12 by means of a bolt 58, thus rigidly securing and grounding the bottom end of the corona rod iti.

A simple and economical construction of the corona rod, corresponding to the schematic indication of FIG- URE l, is shown in FIGURE 3, as used in the embodiment externally illustrated in FIGURES 2 and 4. Each inner annulus 27 is formed of fiberglass roving impregnated with a suitable polyester resin. Each outer annulus 28 is similarly formed. The closed end of the outer cup lli is an annular conducting disc 69, the side wall of the cup being formed by a conducting tube or sleeve 62, preferably woven. The inner cup le is similarly formed from a conducting annular disc 64 and a tube or sleeve 66. This entire assembly is covered with a suitable plastic 68, and plastic spacers 22 between adjacent choke sections make the entire assembly rigid and sturdy. lt

will readily be seen that this construction is fabricated easily and inexpensively, successive insulating and conducting layers of the assembly being built up on the central rod, with appropriate insertion of conducting and insulating annular discs.

in one construction as shown in the drawing, the rod 12 was a 1/4 brass rod 6 feet long, each choke assembly being 13% long with 1A spacers at 22, the conducting annular discs 64 and 60 being soldered to the central rod and being of 7/16 and S/s outer diameter respectively, the sidewalls 62 and 66 being of braided copper tubing, the outer of a length of 13%, as previously stated, and the inner of a length of 131/8, the annular disc being of 1/s" thickness, so that the gap or opening at 2) connecting the two portions of the folded choke was 1/s in length. The discharge point at 24 extended l above the topmost choke. This rod construction was employed with a commercial antenna of the type illustrated known as the Unipole, operating in the range of 42 to 50 megacycles, being mounted at a distance of 61/2" from the grounded conductor, and producing no substantial effect on the circular radiation pattern or the tuning of the antenna, and only minor effects on antenna impedance over the entire tuning range.

Obviously, persons skilled in the art will readily devise constructions which utilize the basic teachings of the invention, but which nevertheless differ substantially in details of construction and operation from the embodiment of the invention herein illustrated. For example, certain aspects of the teachings of the invention may be applied to antenna accessories for purposes other than those of the corona rod illustrated, such as masts for sidemounted antennas. Accordingly, the scope of the invention is not to be limited by the particular embodiment herein described, but is to be determined from the definitions thereof in the appended claims.

What is claimed is:

l. In combination with a vertical-radiator antenna, a corona rod assembly adjacent to and extending beyond the upper end of the vertical radiator and comprising an elongated vertical rod and at least three slightly spaced end-to-end choke assemblies thereon, each choke assem- ,bly comprising a first cup-shaped conductor having the rod extending in tight-litting relation through the closed end thereof and a second cup-shaped conductor of larger diameter inverted with respect to the rst and having the rod extending in tightiitting relation through the closed end thereof, the closed end of the second conductor being slightly spaced from the open end of the rst and the annuli so formed being filled with an insulator of dielectric constant greater than unity.

2. The combination of an antenna and corona rod assembly as set forth in claim 1 wherein the ratio of the diameter of the second cup-shaped conductor to that 0f the irst is smaller than the ratio of the diameter of the first to that of the rod, whereby the exterior surface of thechoke is shortened for any given frequency of resonance.

3. The combination of an antenna and corona rod assembly of claim 2 wherein the annuli are of equal thickness.

4. The combination of an antenna and corona rod assembly of claim l wherein the dielectric constant of the insulating material is at least 4.0.

5. The combination of an antenna and corona rod assembly of claim l wherein the sidewalls of the cupshaped conductors are formed of braided metallic sleeving.

6. The combination as set forth in claim 1 having the upper end of the rod extending beyond the uppermost choke assembly.

7. In combination with a vertical-radiator antenna, a corona rod adjacent thereto comprising a central conductor extending vertically above the end of the radiator and at least three coaxial quarter-wave chokes in closely adjacent end-to-end relation thereon, each choke having an electrical length of one-quarter wavelength and a physical length less than one-eighth wavelength, whereby the assembly acts as a unitary conductor for frequencies other than the resonant frequency, but acts as a series of isolated conductive fragments at the frequency of resonance.

8. The combination as set forth in claim 7 having at least five chokes, each of physical length shorter than one-sixteenth wavelength.

9. Anantenna construction comprising a radiating element having a tuned operating frequency and extending at least partially vertically, and a vertically extending conductor adjacent to the radiating element and having its upper end above the upper end of the radiating element and its lower end grounded, and a plurality of spaced quarter-wave chokes on the conductor, each choke and the spacings therebetween being of an overall length less than one-eighth wavelength at the operating frequency of the antenna.

10. An antenna construction comprising a vertically extending radiator and a corona rod adjacent thereto and parallel therewith and having its upper end extending beyond the upper end of the radiator and its lower end grounded, the corona rod comprising a plurality of coaxial choke assemblies each electrically of a quarterwavelength at the operating frequency of the radiator, and each having an outer conductor having the physical length thereof less than one-eighth wavelength exposed to the radiator, and all having a common axial conductor effectively grounding currents of frequencies other than the operating frequency, all portions of the common axial conductor of greater than one-eighth wavelength being shielded from the radiator.

ll. An antenna construction comprising a vertically extending radiator and a corona rod adjacent thereto and parallel therewith and having its upper end extending beyond the upper end of the radiator and its lower end grounded, the corona rod forming a continuous conductive path for currents of frequencies other than the operating frequency but having thereon choke means isolating successive portions for currents of the operating frequency, all continuous conductor lengths of the corona rod exposed to the radiator being of less than an eighth wavelength.

l2. The antenna construction of claim 11 having grounded conductors forming a counterpoise at the base of the radiator, the corona rod having its lower end joined to the counterpoise.

13. An antenna construction comprising a verticalradiator antenna having conductors forming a counterpoise at the base thereof, and a vertical corona rod extending from the counterpoise to a point above the upper end of the radiator, the corona rod having a plurality of chokes electrically of a quarter wavelength at the operating frequency of the radiator, all continuous conducting surfaces of the corona rod exposed to the radiator being of less than one-eighth wavelength.

14. An antenna construction comprising a radiating element having a tuned operating frequency and extending at least partially vertically, and a vertically extending condguctor adjacent to the radiating element and having its upper end above the upper end of the radiating element and its lower end grounded, and a plurality of quarter-Wave chokes on the conductor, all continuous longitudinally extending portions of the chokes and the conductor which are exposed to the radiator being of less than one-eighth Wavelength.

References Cited in the file of this patent UNITED STATES PATENTS 2,297,513 Von Baeyer Sept. 29, 1942 2,412,640 Varian et al Dec. 17, 1946 2,624,001 Woodward Dec. 30, 1952 2,896,175 Scheldorf July 21, 1959 2,896,206 Scheldorf July 21, 1959 3,031,668 Bryson Apr. 24, 1962 

10. AN ANTENNA CONSTRUCTION COMPRISING A VERTICALLY EXTENDING RADIATOR AND A CORONA ROD ADJACENT THERETO AND PARALLEL THEREWITH AND HAVING ITS UPPER END EXTENDING BEYOND THE UPPER END OF THE RADIATOR AND ITS LOWER END GROUNDED, THE CORONA ROD COMPRISING A PLURALITY OF COAXIAL CHOKE ASSEMBLIES EACH ELECTRICALLY OF A QUARTERWAVELENGTH AT THE OPERATING FREQUENCY OF THE RADIATOR, AND EACH HAVING AN OUTER CONDUCTOR HAVING THE PHYSICAL LENGTH THEREOF LESS THAN ONE-EIGHTH WAVELENGTH EXPOSED TO THE RADIATOR, AND ALL HAVING A COMMON AXIAL CONDUCTOR EFFECTIVELY GROUNDING CURRENTS OF FREQUENCIES OTHER THAN THE OPERATING FREQUENCY, ALL PORTIONS OF THE COMMON AXIAL 